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    SubjectRe: [patch 0/6] sys_indirect RFC - sys_indirect introduction
    On Sat, 30 Jun 2007, Ulrich Drepper wrote:

    > On 6/30/07, Davide Libenzi <davidel@xmailserver.org> wrote:
    > > But, a __get_user(), once you scrap off all the gcc wrapping, is bacially
    > > a move. That could even be removed, but really I don't see the reason
    > > since it allows for a cleaner strcture definition in userland.
    >
    > Don't generalize. The 4G/4G kernel, for instance, doesn't have this
    > property. Who knows what else will come up. Anyway, two memcpy are
    > slower than one.
    >
    > > But this does not allow more than one context to be set at a time, like
    > > the current implementation does. Ie, the current implementation allow you
    > > to:
    >
    > Of course it does. It just requires an appropriate union element.
    > I've listed flags and sigset_t as separate union elements since I
    > cannot think of a place where we need both extensions. Should there
    > be one this can easily be changed.

    Why do you want to stick everything inside a structure, with nested
    strctures and unions, when they clearly are separate contexts?
    Also, unions don't work if you want to pass multiple contexts at a time.
    You need your structure to contains *all* the possible contexts.
    You do not want to sell the monster-struct-union by branding an extra
    __get_user(), do you?
    On top of that, the single pointer to the compat multi-context strcture,
    will force the kernel to decide which one of the members is actually valid
    or not. The set/unset callbacks separate each context from a structure POV
    and from a setup/teardown POV, and allow the kernel to use their native
    data structures. Think about if we had this for the latest
    pselect/ppoll/epoll_pwait. Think about how your solution and mine would
    apply to that very much concrete case.




    - Davide


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